The present invention relates to a new dimple combination pattern on a golf ball surface which improves the flight characteristics of the ball.
According to the United States Golf Association (U.S.G.A.) rules, a golf ball may not have a weight in excess of 1.620 ounces or a diameter smaller than 1.680 inches. The initial velocity of balls conforming to U.S.G.A. regulations may not exceed 250 feet per second with a maximum tolerance of 2%. Initial velocity is measured on a standard machine kept by the U.S.G.A. A projection on a wheel rotating at a defined speed hits the test ball, and the length of time it takes the ball to traverse a set distance after impact is measured. U.S.G.A. regulations also require that a ball not travel a distance greater than 280 yards when hit by the U.S.G.A. outdoor driving machine under specified conditions. In addition to this specification, there is a tolerance of plus 4% and a 2% tolerance for test error.
These specifications limit how far a struck golf ball will travel in several ways. Increasing the weight of a golf ball tends to increase the distance it will travel and lower the trajectory. A ball having greater momentum is better able to overcome drag. Reducing the diameter of the ball also has the effect of increasing the distance it will travel when hit. This is believed to occur primarily because a smaller ball has a smaller projected area and, thus, a lower drag when traveling through the air. Increasing initial velocity increases the distance the ball will travel.
Drag on a golf ball is also reduced by forming a plurality of dimples, often circular, in the outer surface of the ball. The dimples serve to reduce the pressure differential between the front and rear of the ball as it travels through the air.
Numerous dimple configurations for use on golf balls are well-known in the patented prior art, including contoured dimples. For example, the White U.S. Pat. No. 1,418,220 discloses a golf ball having rectangular dimples formed in the surface thereof. Each rectangular dimple includes in its bottom surface a small circular recess. Such a dimple configuration prolongs the life of the golf ball as well as improves the flight of the ball when struck. Golf balls having dimples with different configurations are also known in the patented prior art as shown by the U.S. patents to Nomura et al U.S. Pat No. 4,869,512 and Oka et al U.S. Pat. Nos. 5,143,377, 5,174,578, and 5,338,039. U.S. Pat. No. 5,174,578 for example discloses a golf ball having both circular and polygonal dimples. While the dimple combinations of the prior art increase the turbulence of the air flow across the surface of the golf ball during flight, there is still room for improvement in this regard as a function of dimple designs.
Accordingly, an improved compound dimple was developed as disclosed in the Aoyama U.S. Pat. No. 6,162,136. The compound dimple includes a central depression, an annular depression concentrically surrounding the central depression, and a land ring arranged between the central and annular depressions. In addition, the Kennedy et al U.S. patent application Ser. No. 09/730,868 (which is assigned to the assignee of the present invention) discloses a golf ball with compound and circular dimples. The compound dimples include first and second circular dimple portions with the second dimple portion being arranged in the bottom of the first portion and having a diameter less than that of the first dimple portion.
The present invention was developed in order to improve the aerodynamic efficiency of the dimples for more control of lift and drag of a spinning golf ball during flight.
Accordingly, it is a primary object of the present invention to provide a compound dimple for a golf ball including a first non-circular dimple portion having a first depth and a second non-circular dimple portion at least partially arranged in a surface of the first portion, the second portion having a second depth greater than the first depth.
According to another object of the invention, the second portion is completely arranged within the first portion and has a configuration corresponding to the first portion, with the first and second portions having a common major axis.
According to a further object of the invention, a line along a bottom of the first dimple portion between a surface of the golf ball and a deepest point of the first portion is a straight line.
It is another object of the present invention to provide a golf ball having a spherical surface including a plurality of dimples arranged in the surface. A first group of dimples in the ball surface are compound dimples, each of which includes a first non-circular dimple portion having a first depth and a second non-circular dimple portion at least partially arranged in a surface of the first circular dimple portion, the second portion having a second depth greater than the first depth. A second group of non-circular dimples may also be provided in the golf ball surface.
The compound non-circular dimples are formed by drilling into a spherical surface with a first milling cutter having a first radius during a first milling step, displacing either the first milling cutter or the surface in a first direction during the first milling step to form an elongated first dimple portion, milling into the first dimple portion to a second depth greater than the first depth with a second milling cutter having a second radius less than the first radius during a second milling step, and displacing either the surface or the second milling cutter in the first direction during the second milling step to form an elongated second dimple portion.
The displacement of the second milling cutter during the second milling step can be limited to retain the second dimple portion within the first dimple portion, or the second milling cutter can be displaced relative to the spherical surface so that a portion of the second dimple portion extends beyond the first dimple portion.